With the availability of high-brightness and low-cost LEDs in primary colors (e.g., red, green, and blue), many applications have been developed for a variable color light source. These applications may be purely decorative or may be used for purposes of indicating a status.
The majority of the designs to control these three-color (“RGB”) sources have used digital techniques. Typically, these have used a programmable micro-controller such as a PIC. The brightness of the LEDs has been controlled by pulse width modulation (PWM) or pulse frequency modulation (PFM) techniques. Programmable digital control of LEDs is popular because it offers several advantages in that it is flexible and can be remotely addressed and controlled. An example of such a programmable digital control technique is found in U.S. Pat. No. 6,016,038 to Mueller, et al.
However, digital techniques have several disadvantages as well. The PWF or PFM waveforms used by the controller clocks and the LED control outputs generate substantial electromagnetic interference (EMI) and radio frequency interference (RFI) from audio frequencies up to several Megahertz. Such interference can prevent convenient use of these devices in some applications, or require the use of shielding and grounding to prevent interference with nearby communications and entertainment electronics. EMI regulatory barriers may further increase the expense of using these devices. It is therefore desirable to find ways to control multi-color light sources in a way that reduces EMI and/or RFI interference.
Also, programmable digital controllers are becoming relatively more expensive compared to the LEDs that they are controlling. When blue LEDs were first introduced, they were quite expensive, and thus not suitable for some cost-sensitive applications. The cost of a programmable digital controller was a small fraction of the total system price. However, more recently, the cost of LEDs has dropped substantially, causing the control electronics to represent an increasing fraction of the total device cost. Therefore, the relative expense of programmable controllers has also become a disadvantage, and it is becoming increasingly desirable to find a more cost-efficient alternative.